1. Field of the Invention
This invention relates to charge-coupled device (CCD) field-transfer imager systems incorporating improved smear reduction techniques.
2. Description of the Prior Art
Reference is made to my prior U.S. Pat. No. 4,010,319, entitled "Smear Reduction in CCD Imagers," which issued Mar. 1, 1977, and to my prior U.S. Pat. No. 4,032,976, also entitled "Smear Reduction in CCD Imagers," which issued June 28, 1977. Both of these patents recognize the fact that normally a CCD field-transfer imager continues to be illuminated by a projected image while field-transfer is taking place. This results in smear charge being generated by bright spots in the image.
The obvious solution to this problem is to utilize a shutter to prevent a projected image from reaching the CCD imager during the transfer portion of each successive field. However, in view of the requirement for a motor, a rotating disk or the like and other mechanical parts, with their inherent limitations (power consumption noise, and so on), thus is not a preferred approach.
My U.S. Pat. No. 4,010,319 suggests the use of smear reduction circuitry which is intended to subtract out the smear included with the video at the output of the CCD imager so that a corrected video without significant smear is available for producing a television display of the projected image. The smear reduction technique disclosed in my U.S. Pat. No. 4,010,319 operates very well in substantially reducing all the smear so long as the projected image containing one or more bright spots remains fixed in position. However, when the position of the projected image is moved in a direction perpendicular to the direction of field-transfer (as is the case when a CCD field-transfer imager television camera is panned either left-to-right or right-to-left), the smear reduction circuitry disclosed in my U.S. Pat. No. 4,010,319 is not effective in substantially reducing all of the undesired smear.
My other U.S. Pat. No. 4,032,976 discloses a much simpler and less expensive smear reduction technique than that disclosed in my U.S. Pat. No. 4,010,319. However, it is only capable of reducing a certain portion, but not all, of the smear within each field period. This is true regardless of whether the projected image is fixed or is moving.